Abrasion resistant vascular graft

ABSTRACT

A stent-graft fabricated from a thin-walled, high strength material provides for a more durable and lower profile endoprosthesis. The stent-graft comprises one or more stent segments covered with a fabric formed by the weaving, knitting or braiding of a biocompatible, high tensile strength, abrasion resistant, highly durable yarn such as ultra high molecular weight polyethylene. The one or more stent segments may be balloon expandable or self-expanding. The fabric may be attached to the stent segments utilizing any number of known materials and techniques.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to devices and methods for repairinganeurysms and more particularly, to percutaneously and/or intraluminallydelivered devices and methods for repairing aneurysms such as abdominalaortic aneurysms and thoracic aortic aneurysms.

2. Discussion of the Related Art

An aneurysm is an abnormal dilation of a layer or layers of an arterialwall, usually caused by a systemic collagen synthetic or structuraldefect. An abdominal aortic aneurysm is an aneurysm in the abdominalportion of the aorta, usually located in or near one or both of the twoiliac arteries or near the renal arteries. The aneurysm often arises inthe infrarenal portion of the diseased aorta, for example, below thekidneys. A thoracic aortic aneurysm is an aneurysm in the thoracicportion of the aorta. When left untreated, the aneurysm may rupture,usually causing rapid fatal hemorrhaging.

Aneurysms may be classified or typed by their position as well as by thenumber of aneurysms in a cluster. Typically, abdominal aortic aneurysmsmay be classified into five types. A Type I aneurysm is a singledilation located between the renal arteries and the iliac arteries.Typically, in a Type I aneurysm, the aorta is healthy between the renalarteries and the aneurysm and between the aneurysm and the iliacarteries.

A Type II A aneurysm is a single dilation located between the renalarteries and the iliac arteries. In a Type II A aneurysm, the aorta ishealthy between the renal arteries and the aneurysm, but not healthybetween the aneurysm and the iliac arteries. In other words, thedilation extends to the aortic bifurcation. A Type II B aneurysmcomprises three dilations. One dilation is located between the renalarteries and the iliac arteries. Like a Type II A aneurysm, the aorta ishealthy between the aneurysm and the renal arteries, but not healthybetween the aneurysm and the iliac arteries. The other two dilations arelocated in the iliac arteries between the aortic bifurcation and thebifurcations between the external iliacs and the internal iliacs. Theiliac arteries are healthy between the iliac bifurcation and theaneurysms. A Type II C aneurysm also comprises three dilations. However,in a Type II C aneurysm, the dilations in the iliac arteries extend tothe iliac bifurcation.

A Type III aneurysm is a single dilation located between the renalarteries and the iliac arteries. In a Type III aneurysm, the aorta isnot healthy between the renal arteries and the aneurysm. In other words,the dilation extends to the renal arteries.

A ruptured abdominal aortic aneurysm is presently the thirteenth leadingcause of death in the United States. The routine management of abdominalaortic aneurysms has been surgical bypass, with the placement of a graftin the involved or dilated segment. Although resection with a syntheticgraft via transperitoneal or retroperitoneal procedure has been thestandard treatment, it is associated with significant risk. For example,complications include perioperative myocardial ischemia, renal failure,erectile impotence, intestinal ischemia, infection, lower limb ischemia,spinal cord injury with paralysis, aortaenteric fistula, and death.Surgical treatment of abdominal aortic aneurysms is associated with anoverall mortality rate of five percent in asymptomatic patients, sixteento nineteen percent in symptomatic patients, and is as high as fiftypercent in patients with ruptured abdominal aortic aneurysms.

Disadvantages associated with conventional surgery, in addition to thehigh mortality rate, include an extended recovery period associated withthe large surgical incision and the opening of the abdominal cavity,difficulties in suturing the graft to the aorta, the loss of theexisting thrombosis to support and reinforce the graft, theunsuitability of the surgery for many patients having abdominal aorticaneurysms, and the problems associated with performing the surgery on anemergency basis after the aneurysm has ruptured. Further, the typicalrecovery period is from one to two weeks in the hospital and aconvalescence period, at home, ranging from two to three months or more,if complications ensue. Since many patients having abdominal aorticaneurysms have other chronic illnesses, such as heart, lung, liverand/or kidney disease, coupled with the fact that many of these patientsare older, they are less than ideal candidates for surgery.

The occurrence of aneurysms is not confined to the abdominal region.While abdominal aortic aneurysms are generally the most common,aneurysms in other regions of the aorta or one of its branches arepossible. For example, aneurysms may occur in the thoracic aorta. As isthe case with abdominal aortic aneurysms, the widely accepted approachto treating an aneurysm in the thoracic aorta is surgical repair,involving replacing the aneurysmal segment with a prosthetic device.This surgery, as described above, is a major undertaking, withassociated high risks and with significant mortality and morbidity.

Over the past five years, there has been a great deal of researchdirected at developing less invasive, endovascular, i.e., catheterdirected, techniques for the treatment of aneurysms, specificallyabdominal aortic aneurysms. This has been facilitated by the developmentof vascular stents, which can and have been used in conjunction withstandard or thin-wall graft material in order to create a stent-graft orendograft. The potential advantages of less invasive treatments haveincluded reduced surgical morbidity and mortality along with shorterhospital and intensive care unit stays.

Stent-grafts or endoprostheses are now Food and Drug Administration(FDA) approved and commercially available. Their delivery proceduretypically involves advanced angiographic techniques performed throughvascular accesses gained via surgical cutdown of a remote artery, whichmay include the common femoral or brachial arteries. Over a guidewire,the appropriate size introducer will be placed. The catheter andguidewire are passed through the aneurysm. Through the introducer, thestent-graft will be advanced to the appropriate position. Typicaldeployment of the stent-graft device requires withdrawal of an outersheath while maintaining the position of the stent-graft with aninner-stabilizing device. Most stent-grafts are self-expanding; however,an additional angioplasty procedure, e.g., balloon angioplasty, may berequired to secure the position of the stent-graft. Following theplacement of the stent-graft, standard angiographic views may beobtained.

Due to the large diameter of the above-described devices, typicallygreater than twenty French (3F=1 mm), arteriotomy closure typicallyrequires open surgical repair. Some procedures may require additionalsurgical techniques, such as hypogastric artery embolization, vesselligation, or surgical bypass in order to adequately treat the aneurysmor to maintain blood flow to both lower extremities. Likewise, someprocedures will require additional advanced catheter directedtechniques, such as angioplasty, stent placement and embolization, inorder to successfully exclude the aneurysm and efficiently manage leaks.

While the above-described endoprostheses represent a significantimprovement over conventional surgical techniques, there is a need toimprove the endoprostheses, their method of use and their applicabilityto varied biological conditions. Accordingly, in order to provide a safeand effective alternate means for treating aneurysms, includingabdominal aortic aneurysms and thoracic aortic aneurysms, a number ofdifficulties associated with currently known endoprostheses and theirdelivery systems must be overcome. One concern with the use ofendoprostheses is the prevention of endo-leaks and the disruption of thenormal fluid dynamics of the vasculature. Devices using any technologyshould preferably be simple to position and reposition as necessary,should preferably provide an acute, fluid tight seal, and shouldpreferably be anchored to prevent migration without interfering withnormal blood flow in both the aneurysmal vessel as well as branchingvessels. In addition, devices using the technology should preferably beable to be anchored, sealed, and maintained in bifurcated vessels,tortuous vessels, highly angulated vessels, partially diseased vessels,calcified vessels, odd shaped vessels, short vessels, and long vessels.In order to accomplish this, the endoprostheses should preferably behighly durable, extendable and re-configurable while maintaining acuteand long-term fluid tight seals and anchoring positions.

The endoprostheses should also preferably be able to be deliveredpercutaneously utilizing catheters, guidewires and other devices whichsubstantially eliminate the need for open surgical intervention.Accordingly, the diameter of the endoprostheses in the catheter is animportant factor. This is especially true for aneurysms in the largervessels, such as the thoracic aorta.

SUMMARY OF THE INVENTION

The present invention overcomes the potential disadvantages associatedwith percutaneously delivered endoprostheses as briefly described above.

In accordance with one aspect, the present invention is directed to anendovascular graft. The endovascular graft comprises one or morescaffold structures, a biocompatible, high tensile strength, abrasionresistant, highly durable thin-walled graft material affixed to the oneor more scaffold structures, and at least one connector for connectingthe graft material to the one or more scaffold structures.

In accordance with another aspect, the present invention is directed toan endovascular graft. The endovascular graft comprises a plurality ofindividual stent structures and a graft material formed from an ultrahigh molecular weight polyethylene yarn affixed to an outside portion ofthe plurality of individual stents.

In accordance with another aspect, the present invention is directed toan endovascular graft comprising a substantially tubular structureformed from ultra high molecular weight polyethylene.

The abrasion resistant stent-graft of the present invention comprises atleast one stent segment and a highly durable, abrasion-resistant graftmaterial attached thereto. The graft material may be attached to the atleast one stent segment in any number of ways. The stent-graft may beutilized as a component of a larger system, for example, in a system forrepairing abdominal aortic aneurysms, or as a stand-alone device. Ineither embodiment, the stent-graft is utilized as a fluid carryingconduit that is preferably percutaneously delivered, but may also beutilized surgically. The at least one stent segment may comprise anysuitable scaffold structure and may be fabricated from any number ofbiocompatible materials. The at least one stent segment may beself-expanding or balloon expandable.

The abrasion resistant stent-graft of the present invention ispreferably percutaneously delivered, and as such it is preferablydesigned with the smallest diameter possible. In order to achieve thesmallest diameter possible, thinner graft materials are needed. However,stent-grafts are typically positioned within the body in vessels thathave relatively high hydrodynamic forces, thus requiring graft materialswhich are able to withstand these forces. Essentially, these forces tendto wear the graft material at the points where it is connected to the atleast one stent segment. Over time, the graft material may developmicroleaks which obviously defeat the purpose of the stent-graft,namely, as a by-pass conduit. Accordingly, the abrasion resistantstent-graft of the present invention utilizes a biocompatible, hightensile strength, abrasion resistant, highly durable yarn which may bewoven, knitted or braided into a graft material without sacrificingdiameter.

The yarn or thread may comprise a single component or it may be blendedwith one or more other suitable materials to achieve various desirablecharacteristics, including abrasion resistance, flexibility andthinness. One such yarn comprises ultra high molecular weightpolyethylene, which is commercially available. Accordingly, the abrasionresistant stent-graft of the present invention is a highly durablestent-graft which, because of its thin graft material, may bepercutaneously delivered more easily than present stent-grafts.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages of the invention will beapparent from the following, more particular description of preferredembodiments of the invention, as illustrated in the accompanyingdrawings.

FIG. 1 is an elevational view of an endovascular graft in accordancewith the present invention.

FIG. 2 is a perspective view of an expanded stent segment of theendovascular graft in accordance with the present invention.

FIG. 2A is a fragmentary perspective view of a portion of the stentsegment of FIG. 2.

FIG. 2B is a fragmentary perspective view of a portion of the stentsegment of FIG. 2.

FIG. 2C is an enlarged plan view of a section of the stent segment ofFIG. 2.

FIG. 2D is an enlarged plan view of a section of the stent segment ofFIG. 2.

FIG. 3 is a perspective view of another expanded stent segment of theendovascular graft in accordance with the present invention.

FIG. 4 is an elevational view of an endovascular graft in accordancewith the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to an endovascular graft which may beutilized as a component in a system for use in treating or repairinganeurysms. Systems for treating or repairing aneurysms such as abdominalaortic aneurysms and thoracic aortic aneurysms come in many forms. Atypical system includes an anchoring and/or sealing component which ispositioned in healthy tissue above the aneurysm and one or more graftswhich are in fluid communication with the anchoring and/or sealingcomponent and extend through the aneurysm and anchor in healthy tissuebelow the aneurysm. Essentially, the grafts are the components of thesystem that are utilized to establish a fluid flow path from one sectionof an artery to another section of the same or different artery, therebybypassing the diseased portion of the artery. Current systems arepreferably percutaneously delivered and deployed.

As stated above, the present invention is directed to one component ofan aneurysm repair system; namely, the endovascular graft ofstent-graft. Accordingly, the following detailed description is directedto the endovascular graft. The endovascular graft comprises at least onestent segment and a highly durable, abrasion-resistant graft materialattached thereto. In other words, the endovascular graft of the presentinvention is supported internally by one or more individual stents,which are themselves connected to the graft in a manner which securestheir position, for example, by sutures. It is important to note thatwhile one particular stent design is discussed in detail below, thegraft of the present invention may incorporate any number of suitablestent designs, including self-expanding stents and balloon expandablestents. In addition, the endovascular graft may comprise a device formedsolely from the graft material.

FIG. 1 illustrates an exemplary embodiment of an endovascular graft 10in accordance with the present invention. The exemplary endovasculargraft 10 comprises one or more first stent segments 100, one secondstent segment 200 and a third stent segment 300. In order to illustratethe relationship of the various components comprising the endovasculargraft 10, the endovascular graft is illustrated in the figure as thoughthe graft material were transparent. In a typical use scenario, thethird stent segment 300 would be anchored in healthy tissue below theaneurysm and the uppermost first stent segment 100 would be in fluidcommunication with an anchoring and/or sealing component as brieflydescribed above. It is important to note, however, that depending on thedesign of the system, an anchoring and/or sealing component may not benecessary. The second stent segment 200 comprises a tapered profile,having a diameter at one end equal to that of the first stent segments100 and a diameter at the other end equal to that of the third stentsegment 300. The length of the endovascular graft may be varied by thenumber of first stent segments 100 utilized.

FIG. 2 is a detailed perspective view of an exemplary embodiment of thethird stent segment 300. The third stent segment 300 comprises aplurality of struts 302 connected in a substantially zigzag pattern. Asillustrated, the exemplary third stent segment 300 comprises three setsof zigzag-connected stents 302, thereby forming substantiallydiamond-shaped cells. The non-connected apex 304 of each diamond shapedcell, illustrated in greater detail in FIG. 2A, comprises a smooth,uniform width curved region formed at the intersection of two stents 302of each diamond-shaped cell. This shape is cut directly into the stentsegment 300 during the initial machining steps, typically laser cutting,as is explained in detail subsequently, and is maintained during allsubsequent finishing processing. The junctions 306 between thezigzag-connected stents 302, illustrated in greater detail in FIG. 2Boccurs at the intersection of four struts 302. Preferably, each junction306 of four struts 302 comprises two indentations 308 and 310 asillustrated in FIG. 2B.

The regions proximate the non-connected apexes 304 and the junctions 306are generally the highest stress regions in the third stent segment 300.To minimize the stresses in these regions, these regions are designed tomaintain uniform beam widths proximate where the struts 302interconnect. Beam width refers to the width of a strut 306.Indentations 308 and 310 are cut or machined into the junctions 306 tomaintain a uniform beam width in this area, which is generally subjectto the highest stress. Essentially, by designing the junctions 306 tomaintain uniform beam widths, the stress and strain that would normallybuild up in a concentrated area, proximate the junction 306, is allowedto spread out into the connecting regions, thereby lowering the peakvalues of the stress and strain in the stent structure.

To further minimize the maximum stresses in the struts 302 of the thirdstent segment 300, the struts 302 may have a tapering width. Forexample, in one exemplary embodiment, the struts 302 may be designed tobecome wider as it approaches a junction 306. FIG. 2C is an enlargedpartial view of the third sent segment 300 in its expanded conditionswhich illustrates the tapering width of the struts 302. In thisexemplary embodiment, the strut 302 proximate the junction 306 (width a)is about 0.025 cm and gradually tapers to a dimension of about 0.0178 cmin the mid-region of the strut 302 (width b). By tapering the struts'widths, the stresses in the struts 302 adjacent the junction 306 isspread out away from the junction 306. The tapering of the struts 302 isaccomplished during the machining of the tube of material from which thestent 300 is cut, as described in detail subsequently. However, bytapering the struts 302 in this manner, there is a tradeoff. The stentsegment 300 becomes somewhat less resistant to localized deformations,caused for example, by a protrusion within the vessel lumen. Thislocalized deformation may lead to a local torsional loading on some ofthe struts 302, and, therefore, since the struts 302 in this exemplaryembodiment have a relatively significant portion of their length with areduced width, their torsional rigidity is reduced.

If maximizing the resistance to localized deformation is preferred, thestruts 302 may be maintained at a uniform width, or more preferably havea reverse taper, as illustrated in FIG. 2D, wherein the width at point ais less than the width at point b. In this exemplary embodiment, thereverse taper struts 302 are about 0.025 cm proximate the junction 306and about 0.028 cm in the central region of the struts. While thisreverse taper tends to increase the stresses somewhat proximate thejunctions 306, this increase is very small relative to the decrease instresses gained by having the side indentations 308, 310 illustrated inFIG. 2B, as well as the uniform width connections illustrated in FIG.2A. In addition, since the reverse taper serves to increase thetorsional rigidity of the strut 302, the stent structure resists localdeformation and tends to maintain a substantially circularcross-sectional geometry, even if the lumen into which the stent ispositioned in non-circular in cross-section.

In a preferred exemplary embodiment, the third stent segment 300 isfabricated from a laser cut tube, as described in detail subsequently,of initial dimensions 0.229 cm inside diameter by 0.318 cm outsidediameter. The struts 302 are preferably 0.0229 cm wide adjacent the fourstrut junctions 306 and six mm long, with a reverse taper strut width.Also, to minimize the number of different diameter combination of graftssystems, it is preferred that the third stent segment 300 have anexpanded diameter of sixteen mm. Similarly, the proximal portion of thegraft material forming the legs is flared, having a diameter of sixteenmm. This single diameter for the third stent segment of the graft systemwould enable its use in arteries having a non-aneurysmal region of adiameter from between eight and fourteen mm in diameter. It is alsocontemplated that multiple diameter combinations of third stent segment300 and graft flare would be desirable.

Referring back to FIG. 1, the one or more first stent segments 100 arealso formed from a shape set laser cut tube, similar to the third stentsegment 300 described above. The one or more first stent segments 100comprise a single circumferential row of zigzag or sinusoidally arrangedelements. In the exemplary embodiment illustrated in FIG. 1, and ingreater detail in FIG. 3, the first stent segment 100 comprises tenzigzag or sinusoidal undulations. The one or more first stent segments100 are formed with uniform width connections at the intersections 104of the struts 102 forming the zigzag or sinusoidal pattern. The one ormore first stent segments 100 are preferably cut from tubing having aninside diameter of 0.251 cm and an outside diameter of 0.317 cm. Thestrut widths are preferably about 0.33 cm wide adjacent strutintersections 104 and the struts 102 are preferably seven mm long andthe one or more first stent segments 100 are preferably eleven mm indiameter when expanded.

Referring back to FIG. 1, the second stent segment 200 comprises atapered profile, having a diameter at one end which is the same as theone or more first stent segments 100, and a diameter at the other endmatching the diameter of the third stent segment 300. The second stentsegment 200 is identical to the one or more first stent segments 100except for the taper.

As is explained in detail subsequently, the stent segments 100, 200 and300 are secured in position by the graft material.

The first, second and third stent segments 100, 200, 300 are preferablyself-expandable and formed from a shape memory alloy. Such an alloy maybe deformed from an original, heat-stable configuration to a second,heat-unstable configuration. The application of a desired temperaturecauses the alloy to revert to an original heat-stable configuration. Aparticularly preferred shape memory alloy for this application is binarynickel titanium alloy comprising about 55.8 percent Ni by weight,commercially available under the trade designation NITINOL. This NiTialloy undergoes a phase transformation at physiological temperatures. Astent made of this material is deformable when chilled. Thus, at lowtemperatures, for example, below twenty degrees centigrade, the stent iscompressed so that it can be delivered to the desired location. Thestent may be kept at low temperatures by circulating chilled salinesolutions. The stent expands when the chilled saline is removed and itis exposed to higher temperatures within the patient's body, generallyaround thirty-seven degrees centigrade.

In preferred embodiments, each stent is fabricated from a single pieceof alloy tubing. The tubing is laser cut, shape-set by placing thetubing on a mandrel, and heat-set to its desired expanded shape andsize.

In preferred embodiments, the shape setting is performed in stages atfive hundred degrees centigrade. That is, the stents are placed onsequentially larger mandrels and briefly heated to five hundred degreescentigrade. To minimize grain growth, the total time of exposure to atemperature of five hundred degrees centigrade is limited to fiveminutes. The stents are given their final shape set for four minutes atfive hundred fifty degrees centigrade, and then aged to a temperature offour hundred seventy degrees centigrade to import the proper martensiteto austenite transformation temperature, then blasted, as described indetail subsequently, before electropolishing. This heat treatmentprocess provides for a stent that has a martensite to austenitetransformation which occurs over a relatively narrow temperature range;for example, around fifteen degrees centigrade.

To improve the mechanical integrity of the stent, the rough edges leftby the laser cutting are removed by combination of mechanical gritblasting and electropolishing. The grit blasting is performed to removethe brittle recast layer left by the laser cutting process. This layeris not readily removable by the electropolishing process, and if leftintact, could lead to a brittle fracture of the stent struts. A solutionof seventy percent methanol and thirty percent nitric acid at atemperature of minus forty degrees centgrade or less has been shown towork effectively as an electropolishing solution. Electrical parametersof the electropolishing are selected to remove approximately 0.00127 cmof material from the surfaces of the struts. The clean, electropolishedsurface is the final desired surface for attachment to the graftmaterials. This surface has been found to import good corrosionresistance, fatigue resistance, and wear resistance.

The graft material or component 400, as illustrated in FIG. 4, may bemade from any number of suitable biocompatible materials, includingwoven, knitted, sutured, extruded, or cast materials comprisingpolyester, polytetrafluoroethylene, silicones, urethanes, and ultralightweight polyethylene, such as that commercially available under the tradedesignation SPECTRA™. The materials may be porous or nonporous.Exemplary materials include a woven polyester fabric made from DACRON™or other suitable PET-type polymers.

In one exemplary embodiment, the fabric for the graft material is aforty denier (denier is defined in grams of nine thousand meters of afilament or yarn), twenty-seven filament polyester yarn, having aboutseventy to one-hundred end yarns per cm per face and thirty-two toforty-six pick yarns per cm face. At this weave density, the graftmaterial is relatively impermeable to blood flow through the wall, butis relatively thin, ranging between 0.08 and 0.12 mm in wall thickness.

The graft component 400 is a single lumen tube and preferably has ataper and flared portion woven directly from the loom, as illustratedfor the endovascular graft 10 shown in FIG. 1.

Prior to attachment of the graft component 400 to the stents 100, 200,300, crimps are formed between the stent positions by placing the graftmaterial on a shaped mandrel and thermally forming indentations in thesurface. In the exemplary embodiment illustrated in FIGS. 1 and 4, thecrimps 402 in the graft 400 are about two mm long and 0.5 mm deep. Withthese dimensions, the endovascular graft 10 can bend and flex whilemaintaining an open lumen. Also, prior to attachment of the graftcomponent 400 to the stents 100, 200 300, the graft material is cut in ashape to mate with the end of each end stent.

As stated above, each of the stent segments 100, 200 and 300 is attachedto the graft material 400. The graft material 400 may be attached to thestent segments 100, 200, 300 in any number of suitable ways. In oneexemplary embodiment, the graft material 400 may be attached to thestent segments 100, 200, 300 by sutures.

The method of suturing stents in place is important for minimizing therelative motion or rubbing between the stent struts and the graftmaterial. Because of the pulsatile motion of the vasculature andtherefore the graft system, it is possible for relative motion to occur,particularly in areas where the graft system is in a bend, or if thereare residual folds in the graft material, due to being constrained bythe aorta or iliac arteries.

Ideally, each strut of each stent segment is secured to the graftmaterial by sutures. In an exemplary embodiment, the suture material isblanket stitched to the stent segments at numerous points to securelyfasten the graft material to the stent segments. As stated above, asecure hold is desirable in preventing relative motion in an environmentin which the graft system experiences dynamic motion arising frompulsatile blood pressure, in addition to pulsation of the arteries thatare in direct mechanical contact with the graft system. The stentsnearest the aortic and iliac ends of the graft system (the uppermostfirst stent segment 100 and the third stent segment 300 respectively)are subject to the pulsatile motion arising from direct internalcontact. These struts in particular should be well secured to the graftmaterial. As illustrated in FIG. 4, the stitches 404 on the upper mostfirst stent segment 100 are positioned along the entire zigzagarrangement of struts. The upper and lower apexes of the third stentsegment may be stitched utilizing a similar configuration. It isdifficult to manipulate the suture thread precisely around the strutsthat are located some distance away from an open end, accordingly,various other simpler stitches may be utilized on these struts, or nostitches may be utilized in these areas.

As illustrated in FIG. 4, each of the struts in the first stent segment100 is secured to the graft material 400 which has been cut to match theshape of the stent segment 100. The blanket stitching 404 completelyencircles the strut and bites into the graft material 400. Preferably,the stitch 404 encircles the strut at approximately five equally spacedlocations. Each of the struts on each end of the third stent segment 300is attached to the graft material, which has been cut to make the shapeof the stent segment 300, in the same manner as the first stent segment100.

A significant portion of the graft will not rest directly againstvascular tissue. This portion of the graft will be within the dilatedaneurysm itself. Therefore, this portion of the graft will notexperience any significant pulsatile motion. For this reason, it is notnecessary to secure the stent segments to the graft material asaggressively as the stent structure described above. Therefore, onlypoint stitches 406 are necessary for securing these stents.

It is important to note that a wide variety of sutures are available. Itis equally important to note that there are a number of alternativemeans for attaching the graft material to the stent, including welding,gluing and chemical bonding.

As stated above, In percutaneous procedures, size is a critical factor.One of the more significant determinants of the final diameter of thecatheter system is the bulkiness of the graft material comprising thestent-graft. Accordingly, it is generally accepted that the highestimpact on delivery catheter diameter may be achieved by fabricatingstent-grafts having thinner walls.

Typical stent-grafts are fabricated from a woven polyester and areapproximately 0.005 inches thick. For example, a stent-graft fabricatedfrom a woven polyester low twist, forty denier, twenty-seven filamentyarn having two-hundred thirty yarn ends per inch and one hundred yarnpicks per inch, results in a graft material having a wall thickness ofapproximately 0.005 inches. The graft material is then attached to theinside or outside of a stent or multiple stent segments as describedabove. Appreciable gains may be achieved in having a graft materialthickness in the range from about 0.002 inches to about 0.003 inches.

For a woven graft, as described above, the wall thickness is determinedprimarily by weave density and yarn thickness or bulkiness. It isdesirable to have a graft which is packed tight enough to preventsignificant blood seepage, but not so tight that the yarn bundles pileup on each other. The weaving parameters described above result in justsuch a graft for the particular yarn described. At this density, thegraft material is about as thin walled as it can be without significantpermeability. Also, the yarn described above is only lightly twisted, soas the yarn bundles cross over one another, they tend to flatten out.Higher twisting would both make the graft more permeable and thicker,and the yarn bundle would tend to remain cylindrical at the crossoverpoints. The only remaining parameter that can be utilized to thin thegraft is smaller yarn bundles.

There are two variables which influence yarn bundle size; namely, thenumber of filaments per bundle, and the size or weight of eachindividual filament. The forty denier, twenty-seven filament polyesteryarn described above has a relatively small filament size and arelatively low number of filaments. However, in theory, a much smalleryarn bundle could be contemplated with either few filaments, smallerfilaments, or both. For example, a twenty denier yarn bundle could bemade from fourteen filaments of the same diameter as described above. Ifthis yarn were woven into a graft material with an appropriately denseweave, one would expect a graft material having a thickness ofapproximately 0.0025 inches. While this may work as an acceptable graft,it is possible that the long-term integrity of such a graft may not beacceptable due to the forces described above.

The graft material may be formed utilizing any number of techniques,including weaving, knitting and braiding. Weaving involves theinterlacing, at right angles, of two systems of threads known as warpand filling. Warp threads run lengthwise in a woven fabric and fillingthreads run cross-wise. Knitting is the process of making fabric byinterlocking a series of loops of one or more threads. Braiding involvescrossing diagonally and lengthwise several threads of any of the majortextile fibers to obtain a certain width effect, pattern or style.

A growing concern with a number of endovascular graft systems has beenthat over time, holes may develop in the stent-graft wall, which canlead to blood leakage and possible aneurysm rupture. There is only alimited understanding of the mechanism of hole formation; however, it isgenerally believed to be related to what has been termed chronicmicro-motion between the metallic stent support structures and the graftmaterial. Eventually, this micro-motion may cause the graft material towear away, thereby creating holes.

One potential way in which to overcome this problem is by more tightlybinding the graft material to the stent in areas exhibiting the highestpossibility of micro-motion. There are numerous ways by which the graftmaterial may be attached to the stent, for example, polymeric sutures.Accordingly, it may be possible to simply create a thinner polyestergraft material as described above, more tightly secure it to the stentin areas which exhibit the greatest potential for micro-motion, and havea lower profile, longer wear resistant stent-graft. However, it wouldalso be beneficial to consider alternate materials for fabricating asignificantly thinner graft material with high wear resistance. Higherstrength and/or tougher materials may yield a much thinner stent-graftconduit without sacrificing long-term integrity. In fact, some of thematerials that may be utilized are so much stronger and tougher thanDacron® polyester, that a significantly thinner stent-graft constructedof these materials may be substantially stronger and more wear resistantthan currently available stent-grafts.

There are a number of new, higher performance fibers that aresignificantly stronger and tougher than polyester, and which are alsobiocompatible. Whereas, Dacron® polyester has a tenacity ofapproximately nine grams per denier, many high performance fibers havetenacities in the range from about thirty-five to about forty-five gramsper denier. The more preferred fibers from a strength standpoint forconsideration for use in an ultra thin walled stent-graft material,approximately, 0.002 to 0.003 inches include polyaramid,polyphenylenebenzobisoxazole, liquid crystal polymer and ultra highmolecular weight polyethylene. From a purely strength standpoint, all ofthese materials are suitable for ultra-thin walled stent-graftapplications. However, from a biostability standpoint, ultra highmolecular weight polyethylene fibers may offer a slight advantage in thefact that their basic chemistry is polyethylene, which is known to berelatively inert in biological applications.

Another important consideration for the above-described fibers is theiravailability in fine denier yarns. With current stent-grafts fabricatedfrom a forty denier polymer yarn, it would be difficult to fabricate astent-graft having thinner walls unless the yarn is of a finer denier. Aliquid crystal polymer sold under the tradename Vectran is available asa twenty-five denier yarn. A ultra high molecular weight polyethylenesold under the tradename Spectra is available as a thirty-denier yarn.Another ultra high molecular weight polyethylene sold under thetradename Dyneema is available as a twenty to twenty-five denier yarn.It is also important to consider that ultra high molecular weightpolyethylene fibers only have a density of 0.97 versus 1.38, so that thesame denier yarn would be bulkier in ultra high molecular weightpolyethylene, however, due to the substantial improvement in tensile andabrasive properties, much less ultra high molecular weight polyethylenewould be necessary to obtain equivalent material properties.

Polyethylene is a long chain organic polymer formed by thepolymerization of ethylene. When formed under low pressure, it will formlong polymer chains which increases its resistance to fracture. Ultrahigh molecular weight polyethylene typically has between six and twelvemillion ethylene units per molecule. Ultra high molecular weightpolyethylene has a low coefficient of friction, a high molecular weightand a high density. Accordingly, a fabric made from ultra high molecularweight polyethylene is highly abrasion resistant, highly impactresistant, and highly resistant to damage by water, salt or fresh. Ultrahigh molecular weight polyethylene monofilaments have a high tensilestrength with the associated advantage of stretch resistance andelasticity. These properties make it especially suitable for tortuousbody passageways.

As stated above, polyethylene has a long documented history ofbiocompatability. Given this level of biocompatability, coupled with itsphysical attributes, ultra high molecular weight polyethylene is thepreferred yarn for use as a graft material. The ultra high molecularweight polyethylene yarn may be woven, knitted or braided to form thegraft material and attached to the one or more stent segments asdescribed above. The graft material may also be used as a strand alonedevice for surgical applications or combined with the one or more stentsfor endovascular delivery.

In alternate exemplary embodiments, the ultra high molecular weightpolyethylene yarn may be blended with a dissimilar material, forexample, Dacron® polyester, to manufacture a graft material with alteredbulk properties; e.g., stretch potential, while retaining strength andabrasion resistance. In yet other alternate exemplary embodiment, themonofilament of ultra high molecular weight polyethylene may be blendedtogether with another material to attain a true blended yarn such that afiber or monofilament of one material can be placed next to amonofilament of a second material (third, fourth . . . ) to create aresultant yarn which possesses properties that differ from each of itsmonofilaments.

Although shown and described is what is believed to be the mostpractical and preferred embodiments, it is apparent that departures fromspecific designs and methods described and shown will suggest themselvesto those skilled in the art and may be used without departing from thespirit and scope of the invention. The present invention is notrestricted to the particular constructions described and illustrated,but should be constructed to cohere with all modifications that may fallwithin the scope for the appended claims.

1. An endovascular graft comprising: one or more scaffold structures; abiocompatible, high tensile strength, abrasion resistant, highly durablethin-walled graft material affixed to the one or more scaffoldstructures, the graft material comprising a blend of a monofilament ofultra high molecular weight polyethylene of about twenty-five denier andat least one monofilament of a dissimilar material to create a trueblended yarn that possesses physical and chemical properties that differfrom each of its monofilaments and is configurable into a graft materialwith altered bulk properties; and at least one connector for attachingthe graft material to the one or more scaffold structures.
 2. Theendovascular graft according to claim 1, wherein the one or morescaffold structures comprises a plurality of first stents, a secondstent and a third stent.
 3. The endovascular graft according to claim 2,wherein the graft material is affixed to an outer portion of theplurality of first stents, the second stent and the third stent.
 4. Theendovascular graft according to claim 1, wherein the graft materialcomprises a plurality of crimps between the one or more scaffoldstructures.